The present invention relates to components used in high frequency/microwave circuit applications. Specifically, a single layer gas filled or vacuum capacitor is described for use in millimeter wave applications having a stable capacitance with low radio, frequency signal losses.
Radio communication services are becoming so numerous they are reaching the 50 GHz millimeter wave spectrum. As the demand for more telecommunications services increases, and the spectrum becomes increasingly crowded, it is foreseeable that applications in the 50-100 GHz millimeter wave spectrums will be utilized for various telecommunications applications.
Circuits for generating and processing signals in the millimeter wave spectrum present significant challenges to component designers. As the frequencies increase, the quality of the components becomes increasingly difficult to maintain. Specifically, for a basic capacitor utilized in circuits operating at these frequencies, the internal equivalent series resistance (ESR) increases significantly using known dielectrics and construction techniques for microwave capacitors. Upper frequency spectrum in UHF to SHF are limited because dielectric materials used in the capacitors exhibit a significant change in ESR with frequency. As the frequency increases for a typical high frequency capacitor, the ESR can increase from 0.05 ohm at 200 MHz, to 0.11 ohm at 900 MHz, and to 0.14 ohm at 2,000 MHz and significantly higher ESR at losses can be expected. Additionally, the dielectric constant xcex5 also changes as frequencies increase. Thus, capacitors in particular have a practical upper frequency spectrum at UHF to SHF limitation when they are constructed with conventional dielectric materials.
One of the more advantageous dielectrics is air. Early capacitor designs used in low frequency applications employed air capacitors particularly for high-powered applications. These capacitors were physically large because higher capacitance (20 to 800 pF) are required to work at lower RF frequency (100 KHz to 30 MHz). In order to stand higher working voltage, it was necessary to increase the distance between electrodes. Consequently, the use of air or a vacuum as a dielectric has not seen widespread use outside of this limited application.
Capacitors which utilize a gas, or a vacuum, as a dielectric approach a theoretical performance of an ideal capacitor having no losses and a dielectric constant which remains constant over an extremely wide frequency spectrum up to SHF. The power factor for the earlier low frequency gas-vacuum dielectric capacitors is low, making them suitable for carrying high current levels. In the event of an internal breakdown due to an excessive voltage producing a flash over between capacitor electrodes, the dielectric is self-healing, i.e., it is not destroyed or altered as a result of the arc generated between the electrode plates. Further, as it is known with many dielectric materials used in conventional capacitor applications, a gas or a vacuum dielectric will not suffer from aging and degradation in performance over time.
An additional difficulty in using capacitors of a conventional design at millimeter wave frequencies is that most of these capacitors have a lead wire length, or end cap attachment, which would introduce significant circuit inductance as well as series circuit resistance with the capacitor. In typical microwave applications, the capacitor electrode is directly bonded or soldered to a PCB circuit pattern trace on a circuit board. These connection techniques also introduce the disadvantageous series inductance and resistance.
Accordingly, the present invention is directed to an implementation of a gas filled or a vacuum dielectric capacitor which can be used at extremely high frequencies, up-to and including the millimeter wave spectrum.
A capacitor is provided for high frequency applications. The capacitor utilizes either a vacuum or gas dielectric to provide an improved performance. First and second planer electrodes are separated by micro particles having a diameter in the 3-20 micron range. The micro particles are included in an adhesive spread along the peripheral edges of the planer electrodes. The adhesive bonds the separated electrodes together producing a gas or vacuum dielectric between the electrodes.
In accordance with the preferred embodiment of the present invention, the dielectric may be a vacuum, air, or any number of inert gases which exhibit a dielectric constant approaching 1. The surfaces of the electrodes may be abraded to increase the apparent surface area, and therefore, the nominal capacity of the capacitor.
A method for manufacturing the capacitor permits the dielectric to be either a vacuum or a pressurized gas such as air, nitrogen and gases having superior dielectric properties. In carrying out the method in accordance with the present invention, a sealant mixture containing solid particles is disposed along the periphery of one of the electrodes forming capacitor. After removing air and moisture from the electrodes, the electrodes are pressurized with a dielectric gas. The electrodes are then brought together in a pressure contact with each other so that the sealant joins the electrodes which are spaced apart by the particle spacers within the sealant. Once the sealant has hardened, the capacitor may be removed from the pressurized environment, and the pressure is maintained between the electrodes of the capacitor.